An “oxidation enzyme” is an enzyme that catalyzes one or more oxidation reactions, typically by adding, inserting, contributing or transferring oxygen from a source or donor to a substrate. Such enzymes are also called oxidoreductases or redox enzymes, and encompasses oxygenases, hydrogenases or reductases, oxidases and peroxidases. One such enzyme is galactose oxidase. This invention relates to the selection and production of polynucleotides that encode polypeptides or proteins with biological activity as oxidation enzymes, and in particular galactose oxidase enzymes. These enzymes are produced in facile expression systems such as robust prokaryotic cells (e.g. bacteria) and eukaryotic systems (e.g. fungi and yeast).